Boilers boilers everywhere... System plan correct?

toasty-toes

Hello everyone,
This sure does get deep and complicated in here!
Does anyone have a moment to make sure I am not way off base?

The facts:
Heating degree days 6000
Design temp -22˚F (Yup minus)
960 sq feet of thermal mass in the form of a concrete slab with 4" insulation underneath and 2" around the perimeter.
One zone
4 circuits
Non O2 barrier pex (1/2") in 230 foot loops spaced at 10" - 12" apart.
A little wood stove rated at 30,000 btu/hr

The "almost facts":
My heat loss calculations vary depending on what site or method is used.
Slant fin got me to 26,500
A site called "build it solar" which was far and away the most comprehensive gave me a number of 28,607 (a bit odd as it didn't take into account the passive solar gain. Odd because the site is called "Build it Solar"!)
My guess on this elevated number is due to the fact they ask detailed info about the sq. feet of glazing and I have a slightly higher than norm amount of glazing on the South face of the structure. But it isn't taken into the calculation as a gain when the sun is shining, only a loss.
Now I guess it could be lower than both of these but certainly not higher as I am well insulated.

So, what I was thinking was a Slant/fin Sentry s-34. with a DOE of 29,000 BTU/hr (Not interested in a mod/con). This was about the smallest CI boiler I could come across that I have dealers and installers locally.
A heat exchanger to keep the CI boiler separate from my Non o2 pex.
Have not worked out the pump yet.

Chances I am short cycling with this set-up?
Do I need to add a 20gallon buffer tank?
Am I missing something?
Way off course?
Doomed?

Thanks

toasty-toes

forgot to mention we are at about 2780 feet above sea level.

Zman

All that on you first post? I like it, you have a nice plan!
No, I don't think you need a buffer tank, your boiler has quite a bit of mass and the slab will be very hungry for BTUs. You will have to derate the boiler a bit for the altitude but it still looks big enough.
The one thing you are missing is outdoor reset and condensation protection for the boiler. That slab will drag return temps down and damage the boiler and flue.
If you use a product like this http://www.supplyhouse.com/Taco-I075C3R-1-3-4-3-Way-Outdoor-Reset-I-Series-Mixing-Valve-w-Sensor-5203000-p
One the boiler side, with the mix sensor on the radiant side supply and the return sensor on the boiler side return, you should be good to go.
Carl

toasty-toes

Zman:
Thanks for the vote of confidence and the link to the mixer/reset link. Does this still work if the boiler and slab are separated with a heat exchanger? This is a closed system with no feed water.
What is it mixing in? Now I need to learn about mixing valves and outdoor reset.

Hatterasguy:
Thanks for the info. I guess that makes a lot of sense as well.
Makes you wonder what the point was for all those "properly sized" boilers that were installed.

Experimenting in this kind of climate is not likely to happen. Not to mention there is no way I can afford to have to sell, replace and have installed a new slightly larger boiler. But, for the one week of -22˙F maybe it's not a concern? Maybe its a major concern and I should go to something like the super hot mini gas 50 http://alliedboilers.com/products/mini-gas-boiler/
It was interesting, after reading your post I went back to the slant/fin calculator and went to their suggested boilers and the s-34 wasn't listed. They started at the victory and sentry 60 models. Which seems way too large. But maybe not. ????

What I can tell you was mid winter while we were working on the interior of the place we hooked up a HWT as a temporary heater and it worked well. Even in the -25 weather we had

It was a 40 gal. 32,000 (input) HWT.
I'd have left it there to do the job except it is way too big for the space allotted for boiler "room" and I had to decommission it to finish working in the space when spring came around.

hot_rod

Keep in mind there is usually some "fudge" factor in those load calc programs, some claim as high as 15%. So if you are a tad under the calculated boiler output size, you probably have some wiggle room.

You may be at or under design a lot more of the heating season?

toasty-toes

Hot Rod:
Yes, I figured the online calculators are a pretty rough guesstimate. Nothing really replaces a proper calculation done by a pro, but these seem to be in short supply so I am having to do my homework just to make sure I'm not wasting cash and end up uncomfortable while I do it. I did every calculator I could find and a few manual calculations just to round things out. They all came in at the 24,000 to 28,000 range. Now will I be at or under design all season given the points Hatterasguy brings up?
More typical will be -4˙F for a month or two, but conditions like wind definitely have an effect. Once again one of the most difficult factors to calculate into the mix is the solar gain. My un-scientific observation from last winter was the sun added significantly to the heating of the structure. The make-shift HWT "boiler" simply didn't turn on if it was sunny. And as luck would have it, it is almost always sunny when we are in the deepest of freezes. But I am not counting on this in my calculation as it is too vague and I can't switch on the sun. Unfortunately!

Hatterasguy:
The boiler is in the conditioned space and is slotted to sit probably no more than 3' from where the Pex enters the slab.
The DHW is coming from my On-demand rig I removed from the old house. It works great and we are happy with it.

This is all super interesting as a puzzle. I can see why people get really into it.

hot_rod

The number that will sway the heat load quite a bit is infiltration, is the house fairly tight? Any chance of getting a blower door test done? if so, look for a heat load program that allows you to enter an infiltration number, not just good beer best selection.

There is data available that shows how many days you have been at or below design conditions, usually for the closest weather station. I think it usually goes back 30 years, but who knows what the future holds for temperature conditions at your location.

So you are correct in stating weather including wind, sun, structure integrity, and error in load calc is all part of getting to an accurate number.

I agree with others, if you have wood for backup or additional BTUs, size it close as you have.

toasty-toes

Hot Rod:
Blower door test! hahahaha. Not a chance.
I am a bit out of the main flow for all that stuff.
Even to get a plumber here is a two hour round trip.

The envelope is tight.

hot_rod

The concrete mass in the slab will give you some flywheel. Should be about 12 yards of concrete in that slab, dry weight 35- 3700 lbs per yard, so maybe 44,000 lbs of mass. That would buffer you through some below design days or a power outage

Zman

The mixing valve will work just fine in your application.It just recircs some of the water back through the boiler to keep the temp up.

toasty-toes

Ok, so I am close to the limit with this. It should work if all is correct, but leaves no wiggle room upwards. But I can make up for it in the wood stove. Might still be prudent to get a slightly larger boiler if I can find one.

The air Xchange numbers were from the calculators. Usually denoted as "recent build with all weather stripping etc.."
I built it and I can tell you she is pretty good. Spray foamed walls and vapour barrier till it looked like I was prepping a scene from that TV show "Dexter". Windows detailed correctly, Attic hatch sealed etc etc.

I am asking all this as I have to be a sober second set of eyes for the system design and instal. I have discovered I simply can't trust the folks to do it properly if I am not up to speed myself.

The original guy I got to spec all this out just gave me a blank look when I posed some of these questions. Didn't fill me with confidence.

Zman:
Thanks. I think I get it now. The isolated boiler loop (primary) has this mixer valve on it directing some of the outgoing boiler water directly back to the return and this line bypasses the heat exchanger altogether to keep the differential closer in temp so you avoid condensation. Correct me if I'm wrong.

Zman

You got it. A minor yet important detail.
CArl

toasty-toes

Doesn't sound minor at all!
Thanks.

toasty-toes

Alright, I contacted Slant fin to see if they have any recommended installers within a few hours of here.
Thanks for your input folks.